Fusion GPS co-founder Glenn Simpson, who orchestrated the anti-Trump dossier, provided the Justice Department with a thumb drive in December 2016 containing company’s opposition research into the president.

The disclosure comes from senior Justice Attorney Bruce Ohr in closed congressional testimony last year. Rep. Doug Collins, Republican of Georgia and the ranking House Judiciary Committee member, released a transcript on Friday.

Mr. Ohr’s testimony adds to the picture of wide access Mr. Simpson, a Hillary Clinton opposition researcher, gained to the upper levels of DoJ and the FBI.

Mr. Ohr tells the story of how he became the middleman between the FBI and Mr. Simpson and his contractor, former British spy Christopher Steele. The FBI was investigating the Trump campaign for possible election collusion with Moscow at the time.

The Simpson thumb drive, which Mr. Ohr delivered to the FBI, was his second from Fusion. His wife, Nellie, a Russia expert at Fusion, provided the first one before the election.

In all, Mr. Ohr’s testimony shows that a steady flow of information from Fusion was reaching the FBI at a time when it was making critical decisions pursuing Trump aides.

Mr. Ohr said he didn’t inspect either thumb drive. By December, Mr. Steel had completed a 35-page dossier that contained about a dozen collusion charges, none of which have been proven publicly.

Republicans consider it a hoax and possibly Kremlin disinformation. Mr. Steele said his dossier sources were Russia government officials.

Yet the FBI relied on the dossier at key times, such as using it to convince a judge to let agents wiretap Carter Page, a Trump volunteer, for a year.

Mr. Ohr told a joint House Judiciary-Oversight committee he warned the FBI that Fusion was tied to Mrs. Clinton and therefore they needed to verify the “hearsay.”

“So when I provided it to the FBI, I tried to be clear that this is source information,” Mr. Ohr testified. “I don’t know how reliable it is. You’re going to have to check it out and be aware. These guys were hired by somebody relating to — who’s related to the Clinton campaign, and be aware — you know, they were somehow working associated with the Clinton campaign.”

The FBI did not disclose the Clinton connection in its warrant application. Instead, agents included a footnote that said the bureau “speculates” that Mr. Steele, identified as “source #1,” “was likely looking for information that could be used to discredit Candidate #1 campaign.”

Mr. Ohr began his self-appointed conduit role by meeting in July 2016 with then-FBI Deputy Director Andrew McCabe and his counsel, Lisa Page.

Mr. McCabe, fired for allegedly lying during an inspector general investigation, doesn’t mention Mr. Ohr or the dossier in his best-selling memoir.

This means that Mr. Steele’s anti-Trump conspiracy theories reached the bureau’s highest levels early on. The official Russian probe came later that month, started by agent Peter Strzok.

Mr. Ohr met with Mr. Simpson in August and again after the election. He met with Mr. Steele in July and continued talking for over a year via WhatsApp, Skype and phone calls.

He also conveyed Fusion material to Mr. Strzok and Ms. Page. The two were lovers and shared text messages disparaging Mr. Trump and talking about how to stop him.

“I wanted to get the information, whatever information they had. I wanted to get it to the FBI, and I wanted to try to create the situation for people like Glenn Simpson to talk to the FBI,” Mr. Ohr testified.